Surround sound hum

I just purchased a Sony DAV-DX255, I am getting a humming sound coming from my surround sound speakers and left front speaker. My right front and Sub sound perfect. The left front and right front are the exact same distance from the receiver and when I take the other speaker cables and swap them into the slot of the right front speaker spot they don't humm. I figure that I am doing something wrong, I have always had luck with Sony products, is it possible that 4 of the 6 slots are bad? Thanks a lot

I'm wondering if it's the kind of hum that you can hear from your listening point, or if it's only noticeable when you're right next to the speakers in question. This link (near the bottom) mentions something about that: